1. Field of the Invention
The present invention relates to a vibration generating device, a driving method therefor, a foreign substance removing device, and an optical device. In particular, the present invention relates to a technology that can be applied to various resonant devices of an inkjet head, an oscillatory wave motor, and the like, or a foreign substance removing device for removing foreign substances by vibration, which adhere to a surface of an image pickup apparatus, an optical device, or the like.
2. Description of the Related Art
Piezoelectric materials that can perform bidirectional conversion between electrical energy and mechanical energy are used as various devices for a sonar, a sensor, a speaker, and the like.
Among them, a vibration generating device, which induces vibration in a vibration member using an electrical-mechanical energy conversion element so as to generate a large vibration by resonance at one or more natural frequencies, is used for various resonant devices, a foreign substance removing device, and the like.
For instance, the vibration generating device is used as a resonant device of an inkjet head or an oscillatory wave motor that drives a moving body by vibration induced by the vibration member.
In addition, the vibration generating device is used for a foreign substance removing device for removing foreign substances by vibration, such as dust, dirt, or water droplets that adhere to a surface of an optical component incorporated in an image pickup device such as a digital camera or an optical device, and obstruct an optical path thereof.
A case where the vibration generating device is used in an inkjet head is described as follows. The inkjet head includes a plurality of nozzles constituted of the following members.
Specifically, the nozzle of the inkjet head includes an ink ejection port, an ink chamber filled with ink, a nozzle communicating hole that communicates between the ink chamber and the ejection port, and a vibration member to which a piezoelectric element with electrodes is bonded, and a plurality of such nozzles are disposed.
Further, when a voltage is applied to the piezoelectric element, stress is generated in the piezoelectric element, which causes a pressure change in the ink chamber so that the ink is ejected from the ejection port.
In addition, in the foreign substance removing device for removing foreign substances by vibration, which adhere to a surface of a digital camera or the like, a voltage is applied to the piezoelectric element fixed to the vibration member so as to drive the piezoelectric element for generating elastic vibration that moves the vibration member in the optical axis direction, namely in the thickness direction of the vibration member (hereinafter, referred to as out-of-plane vibration or flexural vibration).
The foreign substance removing device has a structure for removing foreign substances adhered to a surface of the vibration member by the flexural vibration.
In addition, the oscillatory wave motor is also used for driving a camera lens, for example.
Such oscillatory wave motor has a structure in which an alternating voltage is applied to a stator constituted of an electrode and a piezoelectric material that are joined together on the lower surface of the metal vibration member, and a rotor contacted to the upper surface of the stator with friction is driven to rotate by vibration generated in the vibration member.
An amount of static displacement and a displacement rate generated by the piezoelectric adverse effect in the vibration generating device are small, and hence a large voltage is required for generating a desired amount of displacement and displacement rate.
In order to obtain a large voltage, there is known a method of driving by setting a drive frequency to be the same or substantially the same as the natural frequency of the piezoelectric element.
In this way, a resonance phenomenon is generated and enables to generate a large amount of displacement and a high displacement rate in the vibration member even by a low voltage. Thus, drive efficiency can be improved.
The vibration generating device utilizing a resonance phenomenon as described above is called a resonant device. When the resonant device is driven, it is preferred to always drive at the resonance frequency or at a frequency close thereto in a viewpoint of drive efficiency.
However, the resonance frequency varies due to a change in environmental temperature or in a load.
Therefore, it is difficult to always drive by resonance at a constant frequency despite of a change in environment.
For this reason, Japanese Patent Application Laid-Open No. S63-249477 proposes an oscillatory wave motor driving device in which means for sweeping a certain frequency region is used for securely capturing the resonance frequency in driving, so as to securely realize driving at the resonance frequency and to perform stable driving.
In the structure described in Japanese Patent Application Laid-Open No. S63-249477, the drive frequency is swept from high frequency to low frequency so that the drive efficiency at high speed is improved.
In addition, Japanese Patent Application Laid-Open No. 2008-206013 proposes a dust removing device in which a piezoelectric element of vibration generating means for generating vibration in a dust-proof member for preventing dust from adhering to the image sensor is constituted of a piezoelectric ceramic.
Further, Japanese Patent Application Laid-Open No. 2008-206013 discloses that it is conventionally known to use lead titanate zirconate (PZT or PbZr1-xTixO3) as a material of the piezoelectric ceramic constituting the piezoelectric element.
Further, because of the global warming problem caused by CO2 or the surging oil price in recent years, it is demanded to improve efficiency in various devices.
Therefore, the above-mentioned vibration generating device and resonant device for the inkjet head, the foreign substance removing device, the oscillatory wave motor, or the like are required to have higher performance at lower cost. In the above-mentioned conventional example described in Japanese Patent Application Laid-Open No. S63-249477, the drive efficiency is improved by sweeping from high frequency to low frequency, but no specific composition of the piezoelectric material is disclosed for forming the piezoelectric element.
According to our study, the drive efficiency is not always improved by sweeping the drive frequency from high frequency to low frequency, depending on a type of the piezoelectric material. In order to respond to the above-mentioned requirement for high performance in recent years, more improvement is demanded in association with the piezoelectric material.
On the other hand, it is conventionally known to use lead titanate zirconate as the piezoelectric material for forming the piezoelectric material of the piezoelectric element, as described above. In particular, it is conventionally and generally known to use lead titanate zirconate that is designed to have high mechanical quality factor (Qm) for the resonant device.
However, if lead titanate zirconate is used as the piezoelectric material of the resonant device, there is the following problem.
In a resonant device, an amount of vibration displacement is determined by a ratio between vibration rate and drive frequency. In addition, the resonance frequency is determined by a ratio between Young's modulus and density.
Here, the density of lead titanate zirconate (measured by Archimedes' method) is approximately 7.5×103 kg/m3 to 8.0×103 kg/m3, which is high among piezoelectric materials, and the Young's modulus is approximately 60 to 80 GPa, which is smaller than other piezoelectric materials.
In other words, in the case of lead titanate zirconate, the resonance frequency is lower than other piezoelectric materials. As a result, even if the vibration is generated at the same vibration rate, the amount of vibration displacement after conversion is substantially small.
Therefore, the integrated displacement by the sweep is also small, and therefore there is a problem that the drive efficiency is low compared with other piezoelectric materials.
In view of the above-mentioned problem, an object of the present invention is to provide a vibration generating device, a driving method therefor, a foreign substance removing device, and an optical device, in which more improvement of drive efficiency can be realized than the case where the piezoelectric element is constituted of the conventional piezoelectric material of lead titanate zirconate.